Speedometer problem
Thread Starter
Le Mans Master
Joined: Jun 2008
Posts: 8,229
Likes: 411
From: Mt. Sinai NY
Speedometer problem
A friend of mine has a 1992 coupe and is having problems with his speedometer. when he's doing 25 his spedometer is reading 20. At 50 he's doing 35. at 75 he's doing 60 . At 140 he's doing 100. these readings were gathered with the use of GPS and he hasnt changed his tires fopr the wrong size. Can anyone help a guy out ? thank you
clif
clif
Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
If you tell us what gear ratio is in the car we might be able to help, but you have given us very little information. Perhaps a previous owner changed gears and did not install the correct speedometer gears? If that is the case you will need to count driveshaft revolutions to determine the current ratio.
Safety Car
Joined: Jul 2008
Posts: 3,966
Likes: 4
From: Made In America
St. Jude Donor '10-'11
If you tell us what gear ratio is in the car we might be able to help, but you have given us very little information. Perhaps a previous owner changed gears and did not install the correct speedometer gears? If that is the case you will need to count driveshaft revolutions to determine the current ratio.
I had a similar problem and it was with the sensor in the transmission.
Team Owner
Joined: Sep 2003
Posts: 25,383
Likes: 2,736
Clif,
There's something "wrong" with your reported error! There's "likely" no possible way for your "friend's speedo" to reflect errors in 5 mph increments. A rear axle ratio change would reflect a speedometer error that would be an error that would be consistent by a percentage.
An example using your #'s and an 18% numerical axle ratio increase.
Actual speed 20 (GPS) speedometer 23, 35 (GPS) speedometer 41+,
60 (GPS) speedometer 70.8, 100 (GPS) speedometer 118. There are no increments of 5mph.
Your 25/20 reflects 25%, 50/35 reflects 43%, 75/60 reflects 25%, 140/100 reflects 40%.
Drive the car again with the GPS and record the speedometer at known GPS speeds. The more you have the more consistent the calculation for calibration. I'd suggest 20, 30, 40, 50, 60, 70 MPH by GPS. That gives you six records to determine if in fact the error is consistent. If in fact there's no consistency then I'd guess the VSS (Vehicle Speed Sensor) would be a good place to start.
There's something "wrong" with your reported error! There's "likely" no possible way for your "friend's speedo" to reflect errors in 5 mph increments. A rear axle ratio change would reflect a speedometer error that would be an error that would be consistent by a percentage.
An example using your #'s and an 18% numerical axle ratio increase.
Actual speed 20 (GPS) speedometer 23, 35 (GPS) speedometer 41+,
60 (GPS) speedometer 70.8, 100 (GPS) speedometer 118. There are no increments of 5mph.
Your 25/20 reflects 25%, 50/35 reflects 43%, 75/60 reflects 25%, 140/100 reflects 40%.
Drive the car again with the GPS and record the speedometer at known GPS speeds. The more you have the more consistent the calculation for calibration. I'd suggest 20, 30, 40, 50, 60, 70 MPH by GPS. That gives you six records to determine if in fact the error is consistent. If in fact there's no consistency then I'd guess the VSS (Vehicle Speed Sensor) would be a good place to start.
